1. Technical Field
The invention relates to injection molding machines and in particular to a check valve assembly which controls the flow of heated plastic material into a mold cavity. More specifically, the invention relates to an improved check valve assembly which has a spring located within the flow path of the material for biasing the check valve ring toward a material block position to provide a positive valve shut-off for the material flow irrespective of the viscosity of the heated material.
2. Background Information
Injection molding machines of the type in which the invention pertains generally comprise an elongated feedscrew which rotates and reciprocates axially in the cylindrical bore of an elongated heated barrel for moving heated plastic material from an inlet end to the outlet end where it is injected from a distribution chamber through a nozzle and into a mold cavity. The flow of the heated plastic into the distribution chamber for subsequent discharge through the nozzle is controlled by a check valve assembly which includes a movable check ring for regulating the flow of heated material past a front seat and into the distribution chamber.
The check ring moves rearwardly into sealing engagement with a rear seat portion of the check valve assembly in response to back pressure created by the plastic trapped in a filled distribution chamber during the injection molding cycle. When the back pressure is released upon filling of the mold cavity, the incoming plastic moves the check ring away from the rear valve seat allowing for the continued flow of the melted plastic material through the check valve assembly and into the distribution chamber for subsequent discharge through the nozzle and into the mold cavity during the next injection cycle. The heated material is in the form of a cylindrical sleeve as it moves through an annular flow gap formed by the internal periphery of the check ring.
A common problem with such types of check valve assemblies having the movable check ring, is that the viscosity of the plastic material will affect the amount of back pressure created for moving the check ring rearwardly to the material shut-off position. If the check ring is not fully seated in the proper time sequence, additional plastic material may flow past the valve seat affecting the volume of material, and correspondingly affecting the operation of the molding machine and possibly affecting the molded product produced thereby. Thus, it is desirable to provide a positive force for moving the check ring to the material blocking position, which force is independent of the back pressure created by the plastic trapped in the filled distribution chamber, and unaffected by the viscosity of the material.
Examples of prior art injection molding machines utilizing internal springs for various operations of the machine are shown in the following patents.
U.S. Pat. No. 2,940,123 shows an injection molding machine which uses a spring loaded slide valve mounted internally of a needle valve wherein the spring moves the needle valve forward within its channel to close the flow of material through the nozzle orifice overcoming the back pressure exerted by the heated plastic material.
U.S. Pat. No. 3,001,234 discloses the use of a spring in a check shuttle valve which performs a completely different operation than the check valve of the present invention.
U.S. Pat. No. 4,077,756 discloses the use of a spring in the shut-off valve of the mold per se, and not in the check valve assembly of an injection molding machine.
U.S. Pat. No. 4,340,353 discloses the use of a coil spring for the hot sprue valve assembly of an injection molding machine and not in the internal flow control check valve assembly thereof as that of the present invention.
U.S. Pat. No. 4,427,361 shows the use of a coil spring for biasing a mold shut-off pin located within the material distribution chamber and not in the heated material flow path of the check valve assembly of an injection molding machine as in the present invention.
U.S. Pat. No. 4,512,733 discloses the use of a spring mounted within a recess formed in the shank of a valve member for urging the body of the valve member toward a closed position.
U.S. Pat. No. 4,681,528 shows another injection molding machine in which a coil spring is mounted within the material distribution chamber for biasing the closure spool away from the outlet orifice thereof.
U.S. Pat. No. 5,164,204 discloses another shut-off valve for an injection molding machine which utilizes a spring mounted on the shaft of the poppet, out of contact with the molten plastic, for urging the poppet toward the valve body to maintain the valve in a normal closed position.
Although prior art injection molding machines, examples of which are set forth in the above identified patents, utilize springs in their operation, no known prior art machine uses a spring which is located within the flow path of the molten material for biasing the check ring toward the closed position to provide a positive closing action thereon, thereby providing a positive constant closing force on the check ring which move the check ring to the closed position irrespective of the amount of back pressure created thereon, which pressure is affected by the viscosity of the molten material being used in the injection molded machine.